The phase diagrams of the Mg(NO3)(2)-Ln(NO3)(3) (Ln = Nd, Sm, Eu, Tb)-HNO3-H2O systems in the (similar to 20%) HNO3 regions at T= 298.15 K were determined by using an isothermal solution saturation method. The Schreinemaker's wet residues method was used to determine the compositions of solid-phases. Both Mg(NO3)(2)-Nd(NO3)(3)-HNO3-H2O and Mg(NO3)(2)-Sm(NO3)(3)-HNO3-H2O systems are complicated with three equilibrium solid phases Mg(NO3)2 center dot 6H(2)O, Mg(3)Ln(2)(NO3)(12)center dot 24H(2)O and Ln(NO3)3 center dot 6H(2)O (Ln = Nd, Sm). But the systems Mg(NO3)(2)-Eu(NO3)(3)-HNO3-H2O and Mg(NO3)(2)-Tb(NO3)(3)Tb-HNO3-H2O are simple eutonic type. The two new solid-phase compounds Mg3Nd2(NO3)(12)center dot 24H(2)O and Mg3Sm2(NO3)(12)center dot 24H(2)O are congruently soluble in an average medium of-20 mass % HNO3, and they were identified and characterized by the methods of powder X-ray diffraction, X-ray diffraction single crystal structure analysis, and thermogravimetric/differential thermogravimetric. The XRD diffraction patterns for the two new solid-phase compounds are identical due to their isomorphic structure. They belong to hexagonal systems, with space group R3. For Mg3Nd2(NO3)12.24H2O, a= b= (1.0993 0.0005) nm, c= (1.7211 0.0007) nm, V-(1.8012 0.0013) nm3, = (4.2532 0.0024) g.cm(3,) Z=(-)3; for Mg3Sm2(NO3)(12)center dot 24H(2)O, (a = b = 1.1198 0.0004) nm, c= (1.7257 0.0006) nm, V= (1.8742 0.0011) nm(3), = (4.1201 0.0012) g cm 3, Z = 3. The standard molar enthalpies of solution of the compounds Mg3Nd2(NO3)(12)center dot 24H(2)O and Mg3Sm2(NO3)(12)center dot 24H(2)O in water (aq) were measured to be (101.26 +/- 0.77) kJ mol(-1) and (80.27 +/- 0.81) kJ mol(-1), and their standard molar enthalpies of formation were calculated to be- (12214.13 +/- 2.5) kJ mol 1 and- (12183.6 +/- 2.5) kJ mol(-1), respectively. (C) 2019 Elsevier Ltd.